This invention relates to an engine control system and more particularly to an improved arrangement for controlling the ignition timing of an internal combustion engine.
With ignition systems and particularly electronic ignition systems, one of two methods has conventionally been employed for initiating the timing of firing of the spark plug of an internal combustion engine. The spark timing is normally set to occur at a specific crankshaft angle with the actual angle of firing being dependent upon varying parameters of engine operation such as speed, load, etc. The timing angle has been measured as aforenoted by one of two methods.
The first method of determining the spark timing is to use a reference crank angle as a starting point for measuring the timing of the ignition. The time for the engine crankshaft to rotate from this reference angle position to the position at which the spark plug is fired is then computed based upon the average speed of rotation of the crankshaft and the ignition plug is fired at this time interval after the reference crank angle position passes. There is, however, a significant disadvantage to employing such a method. That is, the speed of rotation of the crankshaft actually varies during a given rotation due to a variety of factors. Therefore, the time interval for the crankshaft to move from the reference position to the desired timing position may not be accurately determined by using average crankshaft speed. This can result in poor ignition timing and, therefore, poor or incomplete combustion. In extreme cases, misfiring may actually occur.
Another method of determining the timing is to provide a toothed wheel and a sensor which provides an output signal when each tooth of the wheel passes it. The toothed wheel is either connected to the crankshaft or driven in timed relationship with it. Although such a system provides accurate angular measurement, the accuracy is dependent upon the number of the teeth on the wheel. Also, the timing must coincide with one of the wheel teeth and, this is why the accuracy will depend upon the number of teeth on the wheel. However, as the number of teeth on the wheel increases, the electrical circuitry associated with it becomes more complicated and the entire system becomes more expensive. Also, accuracy can fluctuate particularly as the number of teeth on the wheel is increased due to extraneous electrical noise.
It is, therefore, a principal object to this invention to provide an improved and simple arrangement for determining the ignition timing of an engine.
It is a further object to this invention to provide an arrangement which permits very accurate engine ignition timing with a relatively low cost system